Prepare references to forthcoming crypto example (MystenLabs#4315)

* Prepare references to forthcoming crypto example

* Add link to: https://crates.io/crates/secp256k1/

* Make update from Kostas

* Make Joy's changes

doc/src/explore/examples.md

@@ -11,6 +11,13 @@ Here is a rundown of existing examples in the Sui repository.
 
 In the [Basics](https://github.com/MystenLabs/sui/tree/main/sui_programmability/examples/basics) example, explore object creation, update, and exchange.
 
+## Crypto
+
+In the [Cryptography](https://github.com/MystenLabs/sui/tree/main/sui_programmability/examples/math) example, employ a simple contract to:
+  * Hash a piece of data using keccak256 and output an object with hashed data.
+  * Recover a [Secp256k1](https://crates.io/crates/secp256k1/) signature to its public key and output an object with the public key.
+  * Verify a Secp256k1 signature and produce an event indicating whether it is verified.
+
 ## DeFi
 
 In the [DeFi](https://github.com/MystenLabs/sui/tree/main/sui_programmability/examples/defi) example, find an atomic swap leveraging an escrow agent that is trusted for liveness, but not safety.
@@ -26,4 +33,4 @@ In the [Games](https://github.com/MystenLabs/sui/tree/main/sui_programmability/e
 
 ## NFTs
 
-In the [NFTs](https://github.com/MystenLabs/sui/tree/main/sui_programmability/examples/nfts) example, browse non-fungible tokens of various types and see NFTs representing assets in a game.
+In the [NFTs](https://github.com/MystenLabs/sui/tree/main/sui_programmability/examples/nfts) example, browse non-fungible tokens of various types and see NFTs representing assets in a game.

sui_programmability/examples/README.md

@@ -1,6 +1,7 @@
 Lots of Move code examples, partitioned by category:
 
 * basics: The very simplest examples of Sui programming.
+* crypto: A simple contract to perform ECDSA secp256k1 signature verification and ecrecover (derive the public key from a signature).
 * defi: DeFi primitives like escrows, atomic swaps, flash loans, DEXes.
 * fungible_tokens: Implementations of fungible tokens with different minting and burning policies.
 * games: Various classic and not-so-classic on-chain games.